BAGHDAD — The lone Syrian rebel group with an explicit stamp of approval from Al Qaeda has become one of the uprising’s most effective fighting forces, posing a stark challenge to the United States and other countries that want to support the rebels but not Islamic extremists.

Money flows to the group, the Nusra Front, from like-minded donors abroad. Its fighters, a small minority of the rebels, have the boldness and skill to storm fortified positions and lead other battalions to capture military bases and oil fields. As their successes mount, they gather more weapons and attract more fighters.

The group is a direct offshoot of Al Qaeda in Iraq, Iraqi officials and former Iraqi insurgents say, which has contributed veteran fighters and weapons.

“This is just a simple way of returning the favor to our Syrian brothers that fought with us on the lands of Iraq,” said a veteran of Al Qaeda in Iraq, who said he helped lead the Nusra Front’s efforts in Syria.